1. Field of the Invention
The present invention relates generally to downhole tools run within a subterranean well bore from the well surface during the drilling, completion and production of oil and gas wells and, more specifically, to an improved mechanism for protecting the component parts of such tools during the running-in of such tools to a selected depth within the well bore.
2. Description of the Prior Art
A large number of downhole tools are used in the oil and gas industries which must be lowered from the well surface to a selected location within the well bore. Many of these downhole tools have component parts which are fragile, or which otherwise need protection from scraping, jarring or otherwise impacting the surrounding cased or uncased well bore during the running-in operation. For example, commonly used downhole tools which require the provision of a seal in the annular area located between a fluid transmission conduit, or other tubing string, and a surrounding conduit such as the outer well casing. Such tools may be employed in the drilling and completion of the well, the production of the well, the servicing of the well, or the closing-in of a well. Conventional packers utilize an anchoring system which holds a sealing element in position against other upwardly or downwardly acting pressure differentials in order to establish such an annular seal. Such conventional packers typically employ radially extendible gripping slip systems and radially expandable packing or sealing elements to prevent fluid communication and to provide pressure integrity. Such packers are typically run-in to position within the well bore and set either on a tubing string or on a wireline setting tool. Those packers which are set on a tubing string are typically set using hydraulic pressure within the tubing, hydrostatic pressure in the well bore, or a combination of both. Such packers may also be mechanically set by the application of the force or weight to the tubing string. Permanent packers of the above type include an internal seal bore for receiving tubing which can be retrieved while leaving the packer in place. Retrievable packers utilize such techniques as rotation of the tubing string to release the gripping slip assemblies and packing elements for retrieval of the packing element.
During the above described operations, it is generally necessary that sealing integrity be established between separate elements within the tubing string or between accessory items and the tubing string. Thus, it is generally necessary where a tubing section is inserted into the seal bore of the packer to establish sealing integrity between that section and the packer. One means of providing such sealing integrity is to utilize stacks of sealing elements in which individual sealing elements have a generally chevron-shaped cross-section. Such sealing systems employing chevron-shaped sealing elements are shown, for example on page 62 of the 1990 Baker Service Tools Catalog. These chevron-shaped sealing elements and systems are commonly referred to as tieback accessories or tubing seal systems and are generally employed to establish a seal between a tubing mounted element and the internal seal bore of a conventional packer.
Another method for providing sealing integrity in the tubing casing annulus and to isolate the production zone from portions of the annulus extending above the packing element is to utilize a polished seal bore receptacle in conjunction with sealing elements. Such a seal assembly is again shown on page 62 of the Baker Service Tools 1990 Catalog as the "PBR Tieback Seal Assembly." Such sealing assemblies used in conjunction with polished seal bore receptacles provide a tubing to casing annular seal and permit isolation of the production zone from the tubing-casing annulus. These assemblies can be positioned precisely at a desired location in the casing and permit tubing movement which may result during a production or treating cycle.
There are other advantages associated with the use of seal assemblies of the above type as compared to conventional radially expanding packing elements. Thus, the sealing function can be achieved with a cross-sectional area or gap across which the sealing elements must bridge being much less than that encountered with conventional packing elements. Significant radial expansion of the sealing elements is not required. Also, whereas conventional radially expanding packing elements require a more complicated means of expanding the packing element into sealing relationship, the elements of the tubing seal systems are energized by the pressure which they contain. Thus, they do not need a mechanism to expand them or retain them in sealing relationship.
One problem associated with the use of existing tubing seal assemblies involves damage to the seal means carried on the tubular seal carrier during the running-in operation from the well surface into the polished seal bore receptacle located at the downhole location. The prior art assemblies have not utilized a centralizing device other than a top, stationary gauge ring. Such designs exposed the seals, whether metal-to-metal or elastomeric, to mechanical damage while running into position within the well bore.
The present invention has as its object to provide an improved mechanism for protecting downhole tools during the running-in of such tools from the well surface to a selected downhole location within the well bore or for preventing the premature actuation of such tools during the running-in operation.
In one preferred embodiment, the present invention has as its object to provide a lead-in guide for centralizing a seal assembly being run from a well surface within a well bore to a cooperating sealing bore located at a downhole location.
Another object of the invention is to provide a seal assembly guide for centralizing a packoff seal assembly being run from a well surface within a cased well bore to a cooperating casing or receptacle defining a polished bore surface located at a downhole location.